Andy Coulson admitted his affair with fellow Murdoch newspaper editor Rebekah Brooks was "wrong", as he gave evidence for the first time Monday in Britain's long-running phone-hacking trial.
The former editor of the now-defunct News of the World (NotW), who became Prime Minister David Cameron's communications chief, finally entered the witness box at the Old Bailey court in London, more than five months into the case.
The 46-year-old, married father-of-three acknowledged the "pain" that the on-off relationship with co-defendant Brooks had caused his wife.
"There was an affair that started in 1998. It ended quite soon after but it did re-start," Coulson told England's central criminal court.
"It was not by any means continual. There were very long periods -- very long periods -- where the relationship was what it should have been," he said.
"But I don't want to minimise it or excuse it. It was wrong and it shouldn't have happened and I take my full share of responsibility for the pain it has caused other people, not least my wife."
Coulson denies one charge of conspiracy to hack phones and two of conspiracy to commit misconduct in a public office.
Formerly the showbusiness editor on daily sister tabloid The Sun, Coulson became the NotW weekly's deputy editor in 2000 under Brooks.
He then replaced her as editor in 2003 when she moved up to become chief executive of global media baron Rupert Murdoch's whole British newspaper stable.
Coulson resigned in 2007 after former NotW royal reporter Clive Goodman and private investigator Glenn Mulcaire were convicted of hacking into voicemails.
The former editor told of the culture of secrecy at what was Britain's biggest-selling newspaper, saying the degree of competition between certain desks was "frankly, destructive".
"It seemed crazy that features and news were actively working against each other," he said.
The Sunday paper carried with it a "bit more intrigue, a little bit more secrecy" than The Sun, while reporters had bigger egos and held their contacts "closer to the chests".
- Beckhams cut tabloid 'deal' -
Coulson became Cameron's media advisor in 2007, and the government's communications director in 2010, before resigning in January 2011 as NotW hacking allegations re-surfaced.
Coulson said his family spent a pre-arranged weekend with Cameron after he resigned, in early 2011, but had not spoken to him since.
He said he had had "sparing" contact with Murdoch since his resignation and later did not sit in on meetings between the media mogul and the prime minister.
Discussing the inner workings of the tabloid empire, Coulson said England football icon David Beckham and Spice Girls singer Victoria Adams had cut a deal with The Sun on their engagement announcement in 1998, telling the court the couple "understood how the media works".
"We were the only paper to get the interview. We were the only paper to get the close-up on the ring," he said.
So keen were the Spice Girls to promote their first record that they "turned up at my house", Coulson added.
Asked about the £105,000 ($175,000, 127,000-euro) annual contract for "special inquiries" with Mulcaire's company Nine Consultancy in 2005, Coulson said he did not know the private detective's name until he was arrested alongside Goodman.
"I assumed the description 'special inquiries' meant finding people, looking for people, possibly surveillance," he said, something which was "incredibly important" as "you have got to get there first", but was not an area he was "particularly interested in".
"In the context of a £32-million budget it's not a massive sum," he added.
"We paid double that, I think, to the astrologer (to do the newspaper's horoscopes)."
The trial, which began at the end of October, was originally expected to last three to four months, but may now run into June.
Coulson, Brooks and five other defendants deny all the charges against them.
Coulson resumes giving evidence on Tuesday.